This application claims priority based on EP 00202056.8, filed Jun. 13, 2000.
This invention pertains to a water borne binder composition, a coating composition comprising the same, and a wood substrate coated with said coating composition.
More particularly, this invention pertains to aqueous wood coating compositions and to wood substrates or substrates made from processed wood, such as hardboard, paper or chipboard, with said water borne binder composition.
Environmental legislation is the driving force behind the change from solvent borne coatings to water borne systems. Limits have been established for the amounts of volatile organic compound that are allowed in different coating systems. Until now water borne coatings have found limited acceptance in wood finishing. The use of water borne coatings in the finishing of wood generally has several disadvantages over conventional solvent based coatings, since the water-soluble chromophoric compounds that are present in wood, such as tannins, bleed trough the applied water borne coating to stain and discolor the finish. Furthermore, it is difficult to obtain a satisfactory combination of desirable properties for wood finishes, such as flexibility, surface hardness, sandability, scrub resistance, water whitening resistance, corrosion resistance, and adhesion and blocking resistance. It is especially difficult to obtain a desirable balance between flexibility and proper film formation and hardness/sandability when the amount of volatile organic compounds in the coating composition is limited to 100 g/l.
In attempts to improve the tannin stain blocking of water borne coatings several approaches have been followed, which are described in the patent literature. Reactive pigments generally are quite effective in blocking tannins. However, in practice they have some major drawbacks, since they can cause stability problems such as viscosity increase and polymer gelation or coagulation. Obviously, this solution is limited to pigmented coatings. It is therefore desirable to obtain the tannin blocking properties without the use of reactive pigments, in other words, by modification of the binder composition.
In EP 849,004 an attempt was made to overcome the above-mentioned disadvantages, by proposing a method for the tandem coating of wood substrates. This method comprises the application of two separate coatings, one of them a highly crosslinked coating and the other is a cured coating formed from an aqueous coating composition. The cured coating is formed from an aqueous composition comprising a carbonyl-functional polymer, preferably consisting of ethylene-ureido-containing monomers.
In U.S. Pat. No. 5,141,784 a process is disclosed for treating wood substrates with an effective amount of a carboxylic acid salt and/or a water-soluble compound having one or two salt forming amine groups and having a molecular weight of 50 to about 300,000. In U.S. Pat. No. 4,075,394 the application of an aqueous solution of a polyalkylene imine when treating tannin-containing surfaces is disclosed. Other approaches include the use of cationic binders. The main drawback in that case is the limited availability of paint ingredients that are cationic.
All of the above-mentioned methods suffer from various disadvantages, such as insufficient stability, poor film-forming properties, and insufficient hardness, sandability, scrub resistance, water whitening resistance, corrosion resistance, adhesion and/or blocking resistance, or difficult handling of the coating composition.
It has now been found that by using a combination of a transition metal complex and a composition comprising a copolymer having a specific molecular weight and a specific Tg, coating compositions are obtained that are devoid of the above-mentioned disadvantages and can easily be applied onto wood substrates without the need to use different coating layers. The invention more specifically pertains to a water borne binder composition comprising
a) an aqueous dispersion of an acrylic copolymer comprising carboxylic acid and ethylene-urea functionalities, having a weight average molecular weight below 200,000 and a Tg less than xe2x88x9215xc2x0 C.; and
b) a water-soluble complex of a transition metal.
Stable clear and pigmented water borne coating compositions can be prepared with this binder composition, which can be used advantageously at VOC level s below 100 g/l. After drying the coating compositions give coatings with excellent hardness and good flexibility. These coatings also have a property that prevents the migration of tannins from tannin-containing wood substrates.
According to the present invention, a water borne polymer is provided, preferably prepared from ethylenically unsaturated monomers selected from alkyl esters of acrylic and/or methacrylic acid (denoted as (meth)acrylic acid), such as n-butyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and/or cycloalkyl esters of (meth)acrylic acid, such as isobornyl (meth) acrylate and cyclohexyl (meth)acrylate. Optionally, vinylic monomers such as styrene, vinyl toluene, xcex1-methyl styrene, vinyl acetate, vinyl esters of versatic acids, dienes such as 1,3-butadiene or isoprene, or mixtures thereof, or ethylenically unsaturated nitriles, such as (meth)acrylonitrile, or olefinically unsaturated halides, such as vinyl chloride, vinylidene chloride, and vinyl fluoride can be used in addition to the acrylic monomers.
The monomer composition also contains an ethylenically unsaturated monomer with a carboxylic acid group. Monomers that can be used include (meth)acrylic acid. Optionally, the acid groups can be latently present as, for example, in maleic anhydride, where the acid functionality is present in the form of an anhydride group. Also macro monomers comprising one or more carboxylic acid functional groups can be used. Preferably, monomers such as (meth)acrylic acid are used. Other possible carboxylic acid-functional monomers are oligomerized acrylic acids such as xcex2-carboxyethyl acrylate or its higher analogues (commercially available from Rhodia as Sipomer B-CEA(trademark)), itaconic acid, fumaric acid, maleic acid, citraconic acid, or the anhydrides thereof. Apart from monomers having carboxylic acid functionality also monomers possessing an acid-functional group other than the carboxylic moiety can be present in the monomer composition, such as sulfonic, phosphoric, or phosphonic acid groups, by copolymerizing monomers such as ethylmethacrylate-2-sulfonic acid or the phosphate ester of 2-acrylamido-2-methylpropane sulfonic acid, 2-methyl-2-hydroxyethyl-2-propenoic acid, (1-phenylvinyl)phosphonic acid, or (2-phenylvinyl)phosphonic acid.
The composition furthermore contains an ethylenically unsaturated monomer comprising the ethylene-urea functionality 
The most preferred monomer comprising the ethylene-urea functionality is N-(2-methacryloyloxethyl)ethylene urea: 
Apart from these monomers small amounts of monomers that posses a second functional group may be incorporated into the monomer mixture.
Examples of such monomers are hydroxy-functional monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, (meth)acrylamide or derivatives of (meth)acrylamide such as N-methylol (meth)acrylamide and diacetone acrylamide. Also adducts of hydroxy-functional monomers with ethylene or propylene oxide can be present in the monomer composition
The monomer composition must be chosen in such a way that the overall glass transition temperature of the resulting polymer as calculated using the Fox equation is lower than xe2x88x9215xc2x0 C. (258 K).
The Fox equation, which is well known in the art, is represented by the formula:
1/Tg=W1/Tg(1)+W2/Tg(2)+W3/Tg(3)+ . . .
wherein W1, W2, W3, etcetera, are the weight fractions of the comonomers (1), (2), and (3), (etcetera), and Tg(1), Tg(2), Tg(3) mean the glass transition temperatures of their respective homopolymers.
The amount of monomers with carboxylic acid-functional groups is adjusted to obtain a polymer with an acid value of 15 to 100 mg KOH/g, preferably from 35 to 75 mg KOH/g.
The amount of ethylene-urea functional monomers in the polymer is from 0.1 to 5 wt. %, based on the total of monomers.
The polymer can be prepared by means of emulsion polymerization or by conventional free radical polymerization in an organic solvent followed by emulsification into water and complete or partial removal of the solvent.
The process conditions are chosen in such a way that the resulting polymer has a weight average molecular weight below 200,000, preferably below 150,000.
Preparation of the polymer dispersion by means of emulsion polymerization is preferred.
To the water borne polymer dispersion a water-soluble complex of a transition metal is added. Preferred is a complex containing zinc. Most preferred is zinc ammonium carbonate. In order to ensure stability of the resulting composition, the pH of the polymer dispersion is adjusted to a value above 7 using ammonia or other neutralizing bases. Optionally, a blend of different bases can be used.
The amount of transition metal complex added to the polymer dispersion is chosen in such a way that the ratio between the moles of transition metal ions in the complex and the overall moles of carboxylic acid groups in the polymer is 1:1 to 1:8. Preferably, the ratio is 1:2 to 1:4.
The polymer composition thus obtained can be formulated in clear or pigmented wood coatings that can be used to coat wooden substrates or substrates made from processed wood such as hardboard, paper, chipboard.
The invention is further illustrated by the following examples.